Production of activated carbon exhibiting an elevated catalytic activity

ABSTRACT

Proposed is a process for producing active carbon with increased catalytic activity, wherein the process starts from an active carbon substrate which in particular is an active carbon substrate based on a carbon-bearing cokable raw material, and includes heating of the active carbon substrate to the activation temperature by contacting it with a reaction gas mixture and cooling of the active carbon substrate which is activated in that way.

[0001] The present invention concerns a process for producing active carbon with increased catalytic activity, wherein the process starts from an active carbon substrate. In particular the process according to the invention starts from an active carbon substrate based on carbon-bearing cokable raw materials.

[0002] Active carbon with an increased catalytic activity has been a matter of particular interest in various areas for some time. Active carbon with an increased activity for SO₂ is of great significance for use for keeping air clean, in particular in thin-layer filters, as it adsorption-catalytically converts SO₂ into sulfuric acid in moist air and adsorbs same. In that respect the catalytic activity is to be attributed to the nitrogen component in the active carbon, in which case a controlled introduction of nitrogen into the active carbon during the production thereof has proven to be advantageous, in comparison with the use of nitrogen-rich starting materials.

[0003] U.S. Pat. Nos. 5,352,370, 5,356,849 and 5,444,031 disclose for example processes which start from a fat or pit coal which is subjected to low-temperature carbonization and oxidation, followed by contacting with a small amount of a nitrogen-bearing compound such as for example urea, during the then subsequent calcination and condensation of the carbon structure. That calcination operation is effected at temperatures of between 850 and 950° C. and is carried out in an inert atmosphere. The nitrogen-treated high-temperature coals are then activated to a desired density at temperatures above 700° C. in water vapor or carbon dioxide without the further addition of auxiliary agents such as for example air. The calcined and activated carbon is finally cooled in an oxygen-free atmosphere to temperatures of below 400° C., in which respect cooling to below 200° C. is also disclosed. Lignocellulose materials are mentioned generally as starting materials for the production of the initial active carbon, without those publications involving a more detailed description or discussion of the use of those materials.

[0004] The catalytic reaction of H₂S, SO_(x), NO_(x), peroxides and/or chloramines in liquid and/or gaseous media is referred to as an area of use for the catalytically operative active carbons described in the above-indicated state of the art.

[0005] DE 23 52 790 C3 discloses a process for the removal of sulfur oxides and/or nitrogen oxides from exhaust gases, using activated or post-treated active carbon. That post-treated active carbon was produced by contacting active carbon which according to the disclosure was produced from materials such as charcoal, coke, coconut shells and so forth, with gaseous ammonia, under the effect of heat. It is disclosed in that respect that the ammonia is used in a mixture in an inert carrier gas such as for example nitrogen or flue gas. In that procedure the proportion of inert gas is in the range of between 0 and about 90% by volume with respect to the gaseous ammonia. The presence of other gases which are significant for activation is not disclosed.

[0006] The object of the present invention is to enhance the state of the art at least by a further process for producing active carbon with increased catalytic activity, in particular a process for producing active carbon with increased catalytic activity for SO₂. A further object of the present invention is to provide a process in which as few process steps as possible are required for the production of active carbon with increased catalytic activity, the invention aiming to provide that the process should as far as possible result in a defined product and can be implemented particularly economically.

[0007] According to the invention the present object is attained by a process as set forth in accompanying claim 1. An alternative way of attaining that object is the subject-matter of claim 2. Advantageous embodiments of the process are the subject-matter of appendant claims 3 through 9.

[0008] The process according to the invention is particular advantageous because basically it consists of a single process step comprising three sub-steps which blend continuously into each other. That is already a marked simplification in relation to the processes which are known from the state of the art and in which nitrogen enrichment in the active carbon and the activation procedure are implemented in two process steps which are markedly separate from each other.

[0009] This is possible by virtue of the use according to the invention of a reaction gas mixture for activation and increasing the nitrogen proportion in the active carbon, which at the same time contains NH₃ and CO₂ and/or H₂O. After the increase in the nitrogen proportion in the active carbon has been effected, with simultaneous activation thereof, finally only cooling of the activated active carbon substrate is still required, which is generally implemented in the atmosphere of the above-mentioned reaction gas mixture or under inert conditions.

[0010] In an alternative configuration of the process according to the invention the active carbon substrate to be activated is impregnated with ammonia water so that activation is effected with the metered addition of CO₂ and/or H₂O. This alternative form of the process advantageously provides that a metered addition of gaseous ammonia is redundant and only a slight introduction of steam or water is also required. That considerably reduces the level of complication and expenditure in terms of process engineering.

[0011] Both processes can be carried out without any problem in an electrically heated furnace. In a development of the present invention both alternative forms of the process provide for support heating in the furnace, which is achieved by a gas burning arrangement. As such combustion is effected with air, N₂ is still contained in the reaction gas mixture by virtue of the normal proportion of nitrogen in the air.

[0012] In a particular configuration of the process according to the invention the heating operation is effected by introducing the active carbon substrate into a reactor which is preheated to activation temperature. It is self-apparent in that respect that the active carbon substrate, as moreover throughout the entire duration of the process, is sufficiently moved to ensure uniform contacting of the active carbon substrate with the gas phase and the reactor walls which transfer the required heat. In the investigations which were carried out in the scope of and for the purposes of checking the present invention, it was surprisingly found that, presupposing the usual agitation of the active carbon substrate, a heating rate is set which does not require any expensive regulation and particular checking procedure. In spite of this unexpected simplification in terms of carrying out the process the active carbon produced in accordance with the invention, with increased catalytic activity, is convincing by virtue of its particular catalytic properties.

[0013] In accordance with the invention the activation temperature is between 800 and 970° C. as the increase in the proportion of nitrogen and activation of the active carbon substrate with the reaction gas mixture according to the invention in that temperature range generally results in active carbons with particularly advantageous catalytic and adsorption properties. A temperature range of between 880 and 970° C. is particularly preferred in that respect.

[0014] In a particular configuration of the process according to the invention the gas mixture used for activation of the heated active carbon substrate contains between 1 and 20% by volume of NH₃, between 10 and 60% by volume of H₂O and between 5 and 50% by volume of CO₂, wherein between 1 and 10% by volume of NH₃, between 10 and 30% by volume of H₂O and between 5 and 15% by volume of CO₂ are preferred. The balance is nitrogen. Activation is generally effected over a period of between 30 and 90 minutes, in particular over a period of between 45 and 75 minutes. Activation is generally carried out until reaching the desired catalytic activity for SO₂ or until the reaction for producing an active carbon product with the desired porosity is reached.

[0015] In a particularly preferred configuration of the process according to the invention the active carbon substrate used is an active carbon produced from coconut shells. It is precisely coconut shell-based active carbon that is particularly suitable for the production of active carbon with an increased catalytic activity as it already has from the outset an extremely advantageous pore structure, which surprisingly has a particularly advantageous effect on the increase in the proportion of nitrogen and thus on the level of catalytic activity as well as the adsorptive properties of the product of the process. At the same time that starting material can be commercially obtained in a sufficient amount and inexpensively, with a uniform quality.

[0016] Finally it is preferred if the operation of cooling the activated active carbon substrate is carried out to a temperature <200° C. In the reaction gas atmosphere this ensures that no unwanted change in the activated active carbon substrate occurs by virtue of the oxygen in the air. In addition at a temperature <200° C. the activated active carbon substrate can also be removed without any problem from the reactor being used and passed to intermediate storage or for further processing without the need for particular precautions in terms of temperature resistance of the containers and items of equipment which come into contact with the product of the process.

[0017] The process according to the invention which has been generally described hereinbefore is described in greater detail hereinafter by means of embodiments.

[0018] In the examples described hereinafter, 350 g of active carbon which had been produced from coconut shells was introduced in each case into a metal tube of a diameter of 90 mm and a length of 600 mm. Thereafter the tube was then introduced horizontally into a tubular furnace heated to activation temperature, the temperature being kept constant during the operation of heating the active carbon in the tube. The tube was turned at a speed of about 6 revolutions per minute. After the active carbon had been brought to activation temperature, the gas used to increase the catalytic activity and for activation was passed through the tube and over the active carbon, the tube being removed from the tube furnace together with the active carbon after the expiry of 60 minutes. The product of the process was then cooled in an N₂ atmosphere to a temperature <200° C. Finally the tube was emptied and the active carbon obtained and treated in that way was subjected to suitable analysis.

[0019] The analytical data of the starting material used as the active carbon substrate, based on coconut shells, are as follows: iodine value 670 mg/g specific area in accordance 620 m²/g with N₂-adsorption micropore volume in accordance 0.272 cm³/g with N₂-adsorption H₂O₂-decomposition Δ T after 5 min. 1.1 K Δ T after 10 min. 1.8 K thin-layer bed (2 mm) SO₂-breakthrough after 5 min. 67% after 10 min. 80% after 15 min. 87% reacted SO₂ amount 7 mg/g nitrogen content 0.08%

EXAMPLE 1

[0020] An active carbon substrate was treated in accordance with the foregoing information. A combination of NH₃, CO₂ and H₂O is used as the gas mixture. The gas mixture is obtained by heating a 25% ammonia solution with the simultaneous introduction of CO₂ gas. The activation temperature is in a range of between 900 and 920° C.

[0021] The firing loss of the carbon of the active carbon is a maximum of 16%. By virtue of the change in the surface structure of the active carbon due to the incorporation of nitrogen, wherein the nitrogen content of the active carbon rises from about 0.080% to between 0.3 and 0.4% and the increased firing loss, an increase in the specific surface area, micropore volume, iodine value, reactivity and the adsorption capacity for SO₂ and for n-butane and toluene is achieved.

[0022] The above-specified gas mixture was passed through at the following rate: 36 l/h of NH₃, 102 l/h of H₂O and 70 l/h of CO₂, which results in an overall gas amount of 208 l/h. As a percentage the gas mixture used is composed of 17% by volume of NH₃, 49% by volume of H₂O and 34% by volume of CO₂.

[0023] After cooling the process product had the following properties: iodine value 1130 mg/g specific area in accordance 960 m²/g with N₂-adsorption micropore volume in accordance 0.246 cm³/g with N₂-adsorption H₂O₂-decomposition Δ T after 5 min. 19.0 K Δ T after 10 min. 23.2 K thin-layer bed (2 mm) SO₂-breakthrough after 5 min. 15% after 10 min. 17% after 15 min. 19% reacted SO₂ amount 38 mg/g

EXAMPLE 2

[0024] The procedure used was precisely as in Example 1, except that the gas mixture was passed through at the following rate: 30 l/h of NH₃, 85 l/h of H₂O and 70 l/h of CO₂, which resulted in an overall gas amount of 185 l/h. As a percentage the gas mixture used in this Example was composed of 16% by volume of NH₃, 46% by volume of H₂O and 38% by volume of CO₂.

[0025] The process product obtained in that way had the properties set out hereinafter: iodine value 1040 mg/g specific area in accordance 930 m²/g with N₂-adsorption micropore volume in accordance 0.377 cm³/g with N₂-adsorption H₂O₂-decomposition Δ T after 5 min. 17.5 K Δ T after 10 min. 21.5 K thin-layer bed (2 mm) SO₂-breakthrough after 5 min. 12% after 10 min. 15% after 15 min. 17% reacted SO₂ amount 39 mg/g

EXAMPLE 3

[0026] This Example was also executed as in Example 1, except that the gas mixture was passed through at the following rate: 24 l/h of NH₃, 68 l/h of H₂O and 70 l/h of CO₂, which resulted in an overall gas amount of 162 l/h. As a percentage the gas mixture used in this Example was composed of 15% by volume of NH₃, 42% by volume of H₂O and 42% by volume of CO₂.

[0027] The process product obtained in that way had the properties set out hereinafter: iodine value 1050 mg/g specific area in accordance 840 m²/g with N₂-adsorption micropore volume in accordance 0.349 cm³/g with N₂-adsorption H₂O₂-decomposition Δ T after 5 min. 17.1 K Δ T after 10 min. 20.5 K thin-layer bed (2 mm) SO₂-breakthrough after 5 min. 14% after 10 min. 16% after 15 min. 18% reacted SO₂ amount 37 mg/g

EXAMPLE 4

[0028] Execution of this Example involved using a gas mixture in accordance with Example 1, but the activation temperature was between 830 and 850° C.

[0029] The process product obtained in that way had the properties set out hereinafter: iodine value 1100 mg/g specific area in accordance 890 m²/g with N₂-adsorption micropore volume in accordance 0.380 cm³/g with N₂-adsorption H₂O₂-decomposition Δ T after 5 min. 15.4 K Δ T after 10 min. 21.1 K thin-layer bed (2 mm) SO₂-breakthrough after 5 min. 50% after 10 min. 60% after 15 min. 70% reacted SO₂ amount 16 mg/g

EXAMPLE 5

[0030] Execution of this Example also involved using a gas mixture in accordance with Example 1, but in contrast the activation temperature was set at between 880 and 900° C.

[0031] The process product obtained under these conditions was of the following properties: iodine value 1130 mg/g specific area in accordance 960 m²/g with N₂-adsorption micropore volume in accordance 0.246 cm³/g with N₂-adsorption H₂O₂-decomposition Δ T after 5 min. 18.1 K Δ T after 10 min. 22.6 K thin-layer bed (2 mm) SO₂-breakthrough after 5 min. 15% after 10 min. 17% after 15 min. 19% reacted SO₂ amount 38 mg/g

EXAMPLE 6

[0032] This Example also involved using a gas mixture in accordance with Example 1. The activation temperature set however was between 930 and 950° C.

[0033] The process product obtained under these conditions was of the following properties: iodine value 1110 mg/g specific area in accordance 1080 m²/g with N₂-adsorption micropore volume in accordance 0.377 cm³/g with N₂-adsorption H₂O₂-decomposition Δ T after 5 min. 16.6 K Δ T after 10 min. 21.3 K thin-layer bed (2 mm) SO₂-breakthrough after 5 min. 17% after 10 min. 18% after 15 min. 20% reacted SO₂ amount 37 mg/g

EXAMPLES 7 AND 8

[0034] Examples 7 and 8 were carried out on a technical scale and continuously. In the continuous process for post-activation of coconut shell active carbon active carbon and reaction gas are passed in co-flow relationship. In this case the reaction gases are produced by direct combustion of natural gas in a rotary tube furnace and by a feed of ammonia gas and water vapor. In this procedure the ammonia gas is supplied with the active carbon while the water vapor is additionally injected into the reaction space. In Example 7 activation is effected with NH₃ and H₂O while Example 8 only involves the use of H₂O.

[0035] In practice the rotary tube furnace is pre-heated with natural gas to a temperature of about 920° C. and thereafter the carbon is meteredly introduced jointly with the ammonia gas. Transportation of the active carbon through the furnace is effected by the rotation of the tube which is inclined towards the discharge. The active carbon which has passed through the procedure according to the invention is discharged by way of a pipe coil and simultaneously cooled to a temperature <200° C. The process conditions were as follows: for Example 7 amount of active carbon 35 kg/h temperature (burner regulation) between 900 and 920° C. ammonia gas between 2 and 5 m³ of N₃/h H₂O vapor 16 kg/h reduced pressure <2 mm water column circulatory speed 100 sec/revolution for Example 8 amount of active carbon 35 kg/h temperature (burner regulation) between 900 and 920° C. H₂O vapor 16 kg/h reduced pressure <2 mm water column circulatory speed 100 sec/revolution

[0036] The process products produced in Examples 7 and 8 exhibited the following properties: Example 7 Example 8 iodine value 980 mg/g 990 mg/g specific area in accordance 850 m²/g 820 m²/g with N₂-adsorption 0.346 cm³/g 0.361 cm³/g micropore volume in accordance with N₂-adsorption H₂O₂-decomposition Δ T after 5 min. 11.5 K 2.2 K Δ T after 10 min. 16.4 K 3.5 K nitrogen content 0.35% 0.1% thin-layer bed (2 mm) 41 mg/g 43 mg/g adsorption of n-butane (80 ppm) thin-layer bed (2 mm) 41 mg/g 43 mg/g adsorption of toluene (80 ppm) breakthrough after 5 min. 4% 18% capacity 250 mg/g 250 mg/g thin-layer bed (2 mm) SO₂-breakthrough after 15 min. 14% 53% after 10 min. 16% 66% after 15 min. 17% 78% reacted SO₂ amount 38 mg/g 12 mg/g

EXAMPLE 9

[0037] In a further technical experiment the carbon was saturated prior to the activation procedure with NH₃-water (250%) and introduced into the reactor.

[0038] In this case, at the beginning of the rotary tube furnace, besides the saturated carbon, only the natural gas is also added. Water vapor is added at the center of the reactor or after half the reaction time. amount of active carbon 50 kg/h temperature between 900 and 920° C. NH₃-water 0.3 l/kg/17 l/h H₂O vapor between 20 and 40 kg/h

[0039] Before the addition of the H₂O vapor an intermediate sample 9a was drawn, the end product was identified by 9b.

[0040] Examples 9a and 9b exhibited the following properties: Example 9a Example 9b iodine value 940 mg/g 1070 mg/g H₂O₂-decomposition Δ T after 5 min. 25 K 28 K Δ T after 10 min. 35 K 40 K thin-layer bed (2 mm) SO₂-breakthrough after 5 min. 30% 18% after 10 min. 38% 25% after 15 min. 40% 28% reacted SO₂ amount 30 mg/g 37 mg/g

[0041] It can be seen beyond doubt from the above-described Examples that the process according to the invention surprisingly very easily makes it possible to produce active carbon with increased catalytic activity. In particular, when comparing the properties of the active carbons with increased catalytic activity, produced in Examples 1 through 6, to those of the active carbon substrate used as the starting material, it is found that the use of a reaction gas mixture which contains NH₃, CO₂ and H₂O and optionally N₂, leads to particularly good results in activating and increasing the nitrogen component. The present situation in particular also involves a process which is very simple in terms of process engineering as the active carbon substrate used as the starting material only has to be introduced into a reactor, heated and mixed with the gas mixture.

[0042] Surprisingly it was also found that the active carbons produced in Examples 2 and 3 have a markedly improved adsorption performance in respect of n-butane and toluene, over the active carbon substrate used as the starting material. The values in that respect are set forth hereinafter: Active carbon substrate Example 2 Example 3 thin-layer bed (2 mm) adsorption performance for n-butane immediate breakthrough 30.5% 2.6% 9.8% capacity 19 mg/g 37 mg/g 34 mg/g thin-layer bed (2 mm) adsorption performance for toluene breakthrough 30.2% 7.6% 6.2% after 5 min. capacity 130 mg/g 260 mg/g 230 mg/g

[0043] It becomes particularly clear in this respect that the gas composition according to the invention is the cause of the particular properties of the active carbon produced by the process according to the invention.

[0044] In Examples 4 through 6, the influence of temperature on the properties of the product of the process has been investigated more closely, and it has been found that the best process results are achieved in a temperature range of between 880 and 950° C.

[0045] Examples 7, 8 and 9 serve to clearly illustrate technical implementation of the process in a rotary tube furnace. The properties achieved in that respect in the products of the process are economically readily meaningful and useable so that this provides a process which can already be used without further adaptation on a commercial scale.

[0046] To sum up it is to be noted that the process according to the invention already affords particular advantages over processes from the state of the art by virtue of the reduction in the complication and expenditure in terms of process engineering—basically only one reaction step is still required. In this respect the process remains thoroughly variable and can be adapted to the respective demands made in terms for example of activity of the active carbon or the nature of the starting material used, without involving particular complication and expenditure. 

1. A process for producing active carbon with increased catalytic activity, wherein the process starts from an active carbon substrate, in particular an active carbon substrate on the basis of carbon-bearing cokable raw materials, and includes the following process steps: heating the active carbon substrate to the activation temperature by contacting it with a reaction gas mixture which contains NH₃ and CO₂ and/or H₂O, wherein activation is effected at the same time, and cooling the active carbon substrate which is activated in that way in a reaction gas mixture atmosphere.
 2. A process for producing active carbon with increased catalytic activity, wherein the process starts from an active carbon substrate, in particular an active carbon substrate on the basis of carbon-bearing cokable raw materials, and includes the following process steps: impregnating the active carbon substrate with ammonia water, heating the active carbon substrate to the activation temperature by contacting it with CO₂ and/or H₂O, wherein activation is effected at the same time, and cooling the active carbon substrate which is activated in that way in a reaction atmosphere.
 3. A process as set forth in claim 1 or claim 2 characterized in that the gas or gas mixture used for contacting the active carbon substrate further contains N₂.
 4. A process as set forth in one of claims 1 through 3 characterized in that the heating operation is effected by introducing the active carbon substrate into an activation reactor which is preheated to activation temperature and is concluded when the active carbon substrate has reached the desired activity.
 5. A process as set forth in one of claims 1 through 4 characterized in that the activation temperature is between 800 and 970° C., preferably between 880 and 950° C.
 6. A process as set forth in one of claims 1 through 5 characterized in that the gas mixture used for activation of the heated active carbon substrate contains between 1 and 20% by volume of NH₃, between 10 and 60% by volume of H₂O and between 5 and 50% by volume of CO₂, preferably between 1 and 10% by volume of NH₃, between 10 and 30% by volume of H₂O and between 5 and 15% by volume of CO₂.
 7. A process as set forth in one of the preceding claims characterized in that activation is effected over a period of between 30 and 120 minutes.
 8. A process as set forth in one of the preceding claims characterized in that the active carbon substrate is an active carbon produced from coconut shells, pit coal, fruit stone shell, brown coal coke and polymers.
 9. A process as set forth in one of the preceding claims characterized in that the operation of cooling the activated active carbon substrate is effected to a temperature <200° C. 